The fragment crystallizable (Fc) region links the dual pathogen identification and destruction properties of immunoglobulin G (IgG). Pathogen opsonization positions Fcs to activate pro-inflammatory Fc? receptors (Fc?Rs) on immune cells. Asparagine-linked (N)-glycan attached to Fc is required for productive engagement of the low-affinity Fc?Rs, though it is not known how the Fc N-glycan contributes to Fc?R binding because the N-glycan does not directly contact the Fc?Rs. It has been suggested that the N-glycan provides optimal spacing of two Fc domains, stabilizing the Fc quaternary structure to bind the FC?R. Evidence from our laboratory points to a different hypothesis. We determined that Fc N-glycan motion, increased by Fc amino acid mutations far from the Fc?R binding site, negatively correlated with Fc?RIIIa affinity. Only a single region of Fc, the CE polypeptide loop that contains the site of N-glycosylation, was perturbed as a result of these Fc mutations. This result led to the proposal that the N-glycan affects Fc?R affinity by pre-organizing the CE loop, and not by optimizing Fc domain orientation. Here we will directly test our hypothesis by measuring the structure and motion of the CE loop, in multiple forms stabilized through glycan or protein engineering, using solution nuclear magnetic resonance spectroscopy. The knowledge of CE loop structure and motion will be applied to redesign the Fc polypeptide to generate aglycosylated Fc variants that maintain high affinity for Fc?Rs. The molecular details of immune system activation that will emerge from these studies will be important to understand the process of multiple diseases and will be critical to enhance therapeutic monoclonal antibody function through engineering to treat cancer, transplant and autoimmune disease patients.